1. Field of the Invention
The present invention relates in general to adsorption separation processes, and more particularly to adsorption separation processes carried out in a fixed bed adsorption system comprising at least two beds wherein the fluid employed to cool an adsorption bed after regeneration is different from the fluid feedstock being treated and wherein the end of the cooling step the regenerated bed is filled with cooling fluid and another of the beds is filled with feedstock, said feedstock and cooling fluid being both in the liquid phase. In such processes, the drain and fill steps, conventionally carried out as discrete operations, are combined and integrated as a single displacement step.
2. DESCRIPTION OF THE PRIOR ART
As practiced in accordance with the prior art, an overall process cycle carried out in the liquid phase and using fixed adsorbent beds of particulate adsorbent could comprise as many as six distinct steps, namely:
(a) adsorption of one or more components from a feedstock mixture:
(b) draining the bed of unprocessed feedstock;
(c) regeneration of the bed using a heated purge fluid;
(d) cooling down the newly regenerated bed in preparation for a new adsorption step by passage into the bed of a cooling medium:
(e) draining the cooling medium from the cooled bed; and
(f) filling the void space of the cooled bed with fresh feedstock.
These steps are more fully characterized below:
Adsorption Step (a): During this step the liquid phase feedstock containing the impurities to be removed is passed through a vessel containing a suitable particulate adsorbent such as a zeolitic molecular scene. As the feed passes through the adsorbent bed the impurities (sorbates) are selectively held up by the adsorbent. The feed, now containing significantly less impurity, leaves the adsorption vessel as product. The adsorption step is continued for a fixed time interval or until the impurity levels in the product exceed specifications. At this time the feed is directed to another adsorption vessel of the system, this vessel having been previously regenerated.
Feedstock Drain Step (b): During this drain step, the feedstock remaining in the void space of the vessel at the end of the adsorption step (a) is drained by gravity or pumped out and recycled to feed. If the vessel is drained slowly then the time required for draining will constitute a significant portion of the overall cycle time. If the vessel is drained quickly then the additional flow rate due to material combining with the feed must be considered when sizing the sorbent requirement. In either case, the elimination of the drain step would be of considerable advantage in a liquid phase sorption system.
Regeneration Heat Step (c): After draining step (b), a heated regeneration medium is passed through the adsorbent bed. As the adsorbent is heated it releases the previously sorbed sorbate. The sorbate passes into the regeneration heating medium and is carried out of the system by the latter. The heating step is continued until the bulk of the impurities have been carried out of the adsorption vessel. Regeneration heating is usually carried out with a regenerating medium differing from both product and feedstock.
Regeneration-Cool Step (d): During this step a cooling medium is passed through the hot adsorption vessel to carry out the sensible heat remaining in the adsorption vessel at the end of the regeneration heat step. The cooling is continued until the bulk of sensible heat is carried out of the sorption vessel. In many instances cooling is carried out with a medium other than the feedstock. It is customary to drain this medium before proceeding to the fill step. This adds another step to the overall process cycle.
Cooling Medium Drain Step (e) the step in which the cooling medium remaining in the adsorbent bed void space at the end of Regeneration Cool Step (d) is removed from the bed either by gravity flow or by pumping.
Void Space Filling Step (f): During the fill step, either product or feedstock is used to fill the void spaces in the adsorption vessel before returning the vessel back into service. This is necessary since failure to do so will result in two phase flow and vapor lock. In large volume sorption vessels the time required for filling the vessel can be substantial especially since often the rate at which feed or product is available is often limited.
Upon completion of the fill step the sorption vessel is ready to be put back into the sorption step.
From the preceding description it is apparent that the sorption step must be of long enough duration so that the other sorption vessel(s) can be drained, heated, cooled, and filled before being placed back into service. For such systems sorption processes can become prohibitively expensive and are not competitive with other available separation processes because large sorbent inventories must be used.